The present invention relates to an earthing arrangement and more particularly to earthing arrangements utilised with respect to DC electrical supply systems including electrical supply rails for an electrical power distribution network.
With regard to DC electrical power systems it is known to earth systems at a mid point of the DC power supply. The attached FIG. 4 marked “Prior Art”, illustrates a typical known DC electrical power system 1 in which rails 2, 3 are presented at the exemplary voltages illustrated. An earthing path 4 is created between the rails 2, 3 with a pair of capacitors 5, 6 and an earthing point 7 midway between the capacitors 5, 6. Positioning the earthing point midway between the DC power supply rails reduces the insulation voltage levels in order to avoid corona and partial discharge as well as help minimise the possibility of insulation failure over the lifetime of the system. As indicated, the mid point earth point 7 is created by using two capacitors 5, 6 connected in series with the earthing point 7 tapped between these capacitors 5, 6 which themselves extend between the power supply rails.
With a two power rail or wire DC network the fault condition that practically exists is a short circuit between the two rail lines 2, 3. By earthing the system at the mid voltage level, two more fault possibilities are introduced, that is to say a positive rail 2 to ground fault and a negative rail 3 to ground fault. Under normal conditions, faults involving ground have a far greater probability than rail to rail faults because of insulation arrangements and clearances between the rails 2, 3. In such circumstances in electrical systems earthed at a mid voltage level there is more vulnerability to faults and the majority of these faults involve a short circuit between one rail 2, 3 and earth.